A rating for the ability of a virus to spread. R of 1 would indicate each infected person infects one person. Anything higher than 1 has the potential to grow the number of cases exponentially (i.e. an outbreak or pandemic). For example, measles had an R value of 15.
This is similar to big O notation in estimating computation complexity. R1 would be constant time and R1+ would be O(N^1+).
Links to this note

90% of the Population Needs to Be Vaccinated for Herd Immunity from the Delta Variant
Because the COVID19 Delta variant is much more transmissible (translating to a higher R rate of 50100% higher than a year ago) and mRNA vaccines are 90% effective, we need 90% of the population to be vaccinated to achieve herd immunity. Unfortunately that seems unlikely due to the politicization of the vaccine.

You Don’t Need 100 Bees for 100 Flowers to Bloom
A saying from a Korean drama (which might be poorly translated into English) that all it takes is a few people to spread something. What might seem like a huge effort requires only a few key players e.g. spreading a rumor.

The COVID19 Delta Variant Is 60% More Transmissible Than the Alpha Variant
A mutation of COVID19 is spreading quickly throughout the world and threatens recovery efforts. The Alpha variant is believed to be 50% more transmissible than other local COVID19 types. The Delta variant is 60% more transmissible than the Alpha variant.